Intestinal tight junction dysfunction occurs in a variety of clinical conditions, including food allergies, infections of the gastrointestinal tract, autoimmune diseases, and inflammatory bowel diseases (42). Healthy, mature gut mucosa with its intact tight junction serves as the main barrier to the passage of macromolecules. During the healthy state, small quantities of immunologically active antigens cross the gut host barrier. These antigens are absorbed across the mucosa through at least two pathways. The vast majority of absorbed proteins (up to 90%) crosses the intestinal barrier via the transcellular pathway, followed by lysosomal degradation that converts proteins into smaller, non-immunogenic peptides. These residual peptides are transported as intact proteins, through the paracellular pathway; it involves a subtle but sophisticated regulation of intercellular tight junction that leads to antigen tolerance. When the integrity of the tight junction system is compromised, as with prematurity or after exposure to radiation, chemotherapy, and/or toxins, a deleterious immune response to environmental antigens (including autoimmune diseases and food allergies) may be elicited. There is a continuing need in the art to diagnose and treat such diseases and conditions. There is a continuing need in the art to identify new drugs for treating such diseases.
Several microorganisms exert an irreversible cytopathic effect on epithelial cells that impacts cytoskeletal organization and tight junction function. These bacteria alter intestinal permeability either directly (i.e., EPEC) or through the elaboration of toxins (e.g., Clostridium difficile, Bacteroides fragilis) (43). The Vibrio cholerae phage CXTΦ ZOT protein mimics the human protein zonulin and exploits the physiological mechanisms of tight junction regulation. Zot possesses multiple domains that allow a dual function of the protein as a morphogenetic phage peptide for the Vibrio cholerae phage CTXΦ and as an enterotoxin that modulates intestinal tight junctions. Zot action is mediated by a cascade of intracellular events that lead to a PKCα-dependent polymerization of actin microfilaments strategically localized to regulate the paracellular pathway (38). The toxin exerts its effect by interacting with the surface of enteric cells. Zot binding varies within the intestine, being detectable in the jejunum and distal ileum, decreasing along the villous-crypt axis, and not being detectable in the colon (44). This binding distribution coincides with the regional effect of Zot on intestinal permeability (44) and with the preferential F-actin redistribution induced by Zot in the mature cells of the villi (38).